1. Technical Field
This invention relates to certain polyacetal compositions which are characterized by improved stability during processing. Polyacetal compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxyl terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 weight percent. Compositions based on polyacetals of relatively high number average molecular weight, e.g., 10,000 to 100,000, are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g., compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping and thermoforming. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance. However, in certain applications, it would be desirable to have greater stability during processing than has heretofore been possible with conventional polyacetal compositions or even with polyacetal compositions containing conventional stabilizers. The stabilizer of the present invention imparts such improved stability to the polyacetal resin. The stabilizer of the present invention is a polymer that is non-meltable at the temperature at which the polyacetal is melt processed, has a small number average particle size, both before and after melt processing, and contains formaldehyde reactive nitrogen groups or formaldehyde reactive hydroxyl groups or both.
2. Description of Related Art
Polymers containing nitrogen groups and/or hydroxyl groups have been known in the art to stabilize acetal resins. However, the particular thermal stabilizers of the present invention, which contain formaldehyde reactive hydroxyl groups and/or formaldehyde reactive nitrogen groups (said formaldehyde reactive nitrogen groups being predominantly present on the sides of the polymer backbone), and which also have a small number average particle size and are non-meltable at the temperature at which the polyacetal is melt processed, have not been taught in the art and, in some cases, have been taught away from by the art. In spite of the teachings in the art, which are highlighted below, the particular stabilizers of the present invention significantly improve the stability of polyacetal resins.
Japanese Patent Application Publication 42-7107, published Sept. 11, 1967 by applicant Teijin K. K., discloses a composition of polyoxymethylene or copolymer composition comprising polyoxymethylene polymer or its copolymer and a copolymer of acrylamide and styrene or vinyl napthalene. It is required that the copolymer of acrylamide and styrene or vinyl napthalene have a melting point equal to or below the melting point of the polyoxymethylene polymer or copolymer. The reference teaches away from the use of a non-meltable polymer stabilizer. It also teaches that acrylamide homopolymer is only slightly compatible with polyoxymethylene polymer or copolymer, is not thermally stable, causes considerable discoloration, and has only a slight effect for improving the heat stability or other properties of polyacetal.
Japanese Patent Publication Koho 43-14329, published June 17, 1968 by applicant Mitsubishi Kasei Kogyo K. K., discloses a method of stabilizing formaldehyde polymers consisting of an addition of a copolymer of acrylamide or an acrylamide derivative. There is no teaching that acrylamide homopolymer is a stabilizer for polyacetal, nor is there recognition in this reference of the importance of the non-meltable nature of the stabilizer or of its particle size in achieving improvements in thermal stability of the magnitude achieved herein.
Japanese Patent Application Publication Kohai 59-213752, published May 19, 1983 by applicant Asahi Kosei Kogyo K. K., discloses a polyacetal composition containing a fine poly-beta-alanine powder as a heat stabilizer, wherein the poly-beta-alanine is obtained by hydrogen transfer polymerization of acrylamide. Hydrogen transfer polymerization yields nylon-3, wherein the majority of formaldehyde reactive nitrogen groups are in the backbone of the polymer. The reference teaches away from preparation of a polymer stabilizer having a high degree of formaldehyde reactive nitrogen groups on the sides of the polymer backbone by disclosing that if less than 0.02 moles of the catalyst/mole of acrylamide is used, the vinyl polymer (i.e., polyacrylamide, wherein the formaldehyde reactive nitrogen groups are on the sides of the polymer backbone) tends to be generated at an undesirable degree. Further, there is no recognition of the particular parameters required for the polymer stabilizers of the present invention. Even further, in an application later filed by Asahi (i.e., West German Published Application No. P3715117.7, discussed below), it is stated that the stabilizers of this reference are still inadequate with regard to heat resistance and the absence of discoloration under heat.
West German Published Application No. P3715117.7, published Dec. 3, 1984 by Asahi Kasei Kogyo K. K., discloses a thermal stabilizer for polyacetal purported to be improved over that of the Japanese 59-21375 reference. In this particular reference, there is disclosed as a thermal stabilizer for polyacetal a poly-beta-alanine compound containing: ##STR1## characterized in that the content of monomer units (Y) lies in the range from 1.4 to 10 mM per gram of the compound, preferably from 5 to 9 mM per gram of the compound. This reference clearly teaches away from a high percentage of (Y) monomer units, as evidenced by the preferred range of (Y) monomer units and by the statement that excellent characteristics cannot be obtained if poly-B-alanine (corresponding to poly-X) is merely mixed with a polyacrylamide (corresponding to poly-Y). This indicates that poly-Y, by itself, is not effective without interaction from poly-X. The reference also does not recognize the importance of the particular parameters of the present invention.
European Patent Application Publication No. 0 245 962, published Nov. 19, 1987 by applicant E. I. du Pont de Nemours and Co., Inc. (inventor E. R. Novak) discloses a thermal stabilizer for polyacetal consisting of polymers and oligomers containing both hydroxyl groups and at least one other functional group that is a stronger Lewis base than the hydroxyl groups. Excessively high stabilizer viscosity (from molecular weight or chemical composition) is undesirable because it can make it difficult to disperse the stabilizer in the polyacetal. U.S. Pat. No. 4,766,168, issued Aug. 23, 1988 to N. E. West and assigned to E. I. du Pont de Nemours and Co., Inc. discloses a thermal stabilizer for polyacetal consisting of hydroxy containing polymers and hydroxy containing oligomers. Neither reference recognizes the importance of the stabilizer particle size and the non-meltable nature of the stabilizer with respect to obtaining thermal stability of the magnitude demonstrated herein.
U.S. Pat. No. 3,204,014, granted Aug. 31, 1965 to Green, discloses a thermally stable polyacetal composition comprising a polyacetal and a ternary stabilizer system comprised of (a) a copolymer of N-vinylpyrrolidone and a monoolefinic monomer copolymerizable therewith, (b) a phenolic antioxidant, and (c) a diester of 3,3'-thiodipropionic acid. There is no recognition of the importance of the average particle size or the non-meltable nature of the stabilizer.
U.S. Pat. No. 3,219,727, granted Nov. 23, 1965 to Kray, discloses polyoxymethylene compositions having enhanced heat stability due to the incorporation therein of a vinyl polymer having side chains containing the structure ##STR2## and particularly containing groups of the class consisting of amide groups and lactam groups. Although there is disclosed in this reference stabilizers for polyacetal containing nitrogen groups, there is no recognition of the importance of small number average particle size in this reference, nor is there recognition that the stabilizer should be non-meltable, nor is there exemplified any of the particular stabilizers of the present invention, said stabilizers having a small number average particle size and being non-meltable.
U.S. Pat. No. 3,235,531, granted Feb. 15, 1966 to Walker, discloses a method for stabilizing oxymethylene copolymer. U.S. Pat. No. 3,210,322, granted Oct. 5, 1965 to Polly discloses a method of treating a trioxane polymerization product comprised of an oxymethylene polymer, a trioxane polymerization catalyst, and unreacted trioxane. U.S. Pat. No. 3,236,929, granted Feb. 22, 1966 to Jupa, discloses a method of injection molding oxymethylene copolymers. U.S. Pat. No. 3,418,280, granted to Orgen, discloses a method of stabilizing a polymer which is susceptible to thermal degradation because it contains monomeric units susceptible to degradation. In the above four references, the oxymethylene polymer or copolymer may contain thermal stabilizers such as substituted polyacrylamides or compounds having 1-6 amide groups. None of the references recognize the particular parameters of the present invention, nor do any exemplify the particular stabilizer of the present invention.
U.S. Pat. No. 4,230,606, granted Oct. 28, 1980 to Amann, discloses a melamine-formaldehyde polycondensate as a thermal stabilizer for polyoxymethylene. It is advantageous to form the melamine-formaldehyde precondensate in the presence of one or more protective colloids, which may include copolymers of acrylamide. There is no suggestion that copolymers of acrylamides are stabilizers for polyacetal.
None of the references discussed or listed above disclose the particulars of the present invention nor do any exemplify the specific stabilizers claimed herein.